Acting for Two: Elsa Lanchester plays both Mary Shelley and the Bride.
Deleted Scene: James Whale cut out a few scenes before release, trimming the movie down from 90 minutes to 75; this included a re-shot ending where Dr. Frankenstein survives (he was originally meant to die in the laboratory explosion) and some bits that were excised at the behest of the censors, like a scene where Dwight Frye's character murders his uncle and blames it on the monster, or some shots of Elsa Lanchester that were deemed too revealing. Don't expect to ever see these scenes.
Executive Meddling: Due to the Hays Code now being fully in effect, they were now more limited on the violence and number of deaths they could show.
Leaning on the Fourth Wall: The final note of the musical score, played just as the laboratory explodes, is a very powerful dissonant chord, made to be so loud that it would slightly shake the seats in the movie theater. Whale came up with this idea, and meant for it to suggest that the explosion was so massive that it even affected the real world. This element is, unfortunately, mostly lost when watching it on home video.
Not Named in Opening Credits: The opening credits bill "The Monster's Mate" with a question mark, and only gives Elsa Lanchester credit for playing Mary Shelley in the prologue.
The Other Darrin: The actors for Elizabeth, the Burgomaster, and Hans are replaced.
Science Marches On: "The human heart is more complex than any other part of the body". Apparently growing brains in a vat is easier than we think.
Stock Footage: Footage from the first film is shown while Lord Byron is remembering the story.
What Could Have Been: Original plans for a sequel to Frankenstein were very different. Two other possible plots were conceived, one being the Monster continuing Dr. Frankenstein's research and the other one being Dr. Frankenstein inventing a death ray in the eve of World War I.
The role of Dr. Pretorius was offered to Claude Rains, but he was attached to another project at the time and couldn't participate. They also offered it to Bela Lugosi, but he turned it down.
The film was initially planned to be called The Return of Frankenstein.